It will usually cost you $500 in legal fees and around $200 in court filing fees.

Should I require an upfront fee, a contract, etc.

I also put lease break fees into my lease for situations like this but if the sisters are willing to cover the whole month, you may luck out and not lose any money or have to go to court at all. 

@Wayne BraultI know some brokers offer a flat fee listing on the MLS.

However if I factor in property management cost and gardener fee, I am in the hole closer to $300 a month.

In North Carolina, it's customary to have a "due diligence" fee when executing a purchase agreement.

Be sure to use the OK language for when rent is late, late fees and other rules that are often dictated by state law.  

When you submit your offer, make sure you have a due diligence period that gives you enough time to do these inspections.Once you get the property under contract, have the septic company do their inspection, have an engineer visit if necessary and get your property inspector out to do a full property inspection -- if he is concerned about a major plumbing issue or mold issue, you can get those specialty trades out to take a look and give you an estimate.If it turns out there is a bunch of extra work in these areas (or ANY areas) that you didn't expect, you go back to the seller and basically say, "Hey, there were hidden defects that I needed to bring in specialty contractors to evaluate, and here are the reports they provided. 

Our option fee is $3900 to $10,000 depending on the price point of the property.

Obviously speak to an attorney, but document all fees for cleaning, and take from the security deposit.